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Dozens of viruses aren’t causing positive COVID-19 tests

January 25, 2022 GMT

CLAIM: COVID-19 PCR test instructions show a “list of the viruses that can cause a PCR test to give false positives.”

AP’S ASSESSMENT: False. The list being cited on social media shows viruses and other pathogens that are used to ensure the COVID-19 tests are accurate for detecting only the coronavirus.

THE FACTS: Recent social media posts are sharing a list of viruses and other pathogens they wrongly claim can trigger false positive COVID-19 results on PCR tests. But the posts are misinterpreting technical information for the tests.

One widely shared Facebook post lists more than 50 “viruses that can cause a PCR test to give false positives.” Citing a test’s “instruction leaflet,” the post lists H1N1, measles virus, influenza B Yamagata and more.

It’s unclear which of the many PCR tests available is being referenced. But some posts do also include a screenshot from one test’s instruction for use that contains a similar list — ignoring that the document says that an analysis showed that “a false positive result is not likely.”

COVID-19 PCR, or polymerase chain reaction, tests are highly specific and look for genetic material of the coronavirus to determine whether it is present in a sample.

Stephanie Caccomo, an FDA spokesperson, said such claims that dozens of other viruses cause false positive results on COVID-19 PCR tests are “inaccurate.”

“For EUA authorization, cross-reactivity studies are required (as noted in our template for test developers) to demonstrate that COVID-19 PCR diagnostic tests do not react with related pathogens, high prevalence disease agents, and normal or pathogenic flora that are reasonably likely to be encountered in a clinical sample,” Caccomo said in an email.

Instead, Caccomo noted, various tests’ instructions for use include a list of the pathogens evaluated for cross-reactivity to ensure that they do not trigger a false positive.

It is “standard laboratory medicine practice to confirm the specificity of a test by challenging it with other similar compounds,” said Nam Tran, senior director of clinical pathology at the University of California, Davis, who serves on the California COVID-19 testing task force. “We do this for every test long before the pandemic.”

Dr. Bobbi Pritt, chair of the division of microbiology at the Mayo Clinic, said the social media posts appeared to simply misinterpret what test documents said.

“If there’s a person who has any of these viruses” and was tested for COVID-19, “they would get a negative result,” she said.

Pritt said tests authorized for emergency use in the U.S. must meet strict criteria for accuracy.

Experts say false positives with PCR tests are rare, though still possible with any test. Still, social media posts have long used false claims to allege that false results are responsible for high COVID-19 case numbers.

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This is part of AP’s effort to address widely shared misinformation, including work with outside companies and organizations to add factual context to misleading content that is circulating online. Learn more about fact-checking at AP.